One of the problems associated with mechanisms such as reciprocating piston engines is that of balancing inertia shaking forces and moments that act on the engine supports due to the moving masses. These shaking forces and moments may cause vibration with accompanying noise and stress on various parts of an engine and its mountings.
The various types of unbalance forces existing in engines having various cylinder arrangements have been extensively analyzed in the prior art. For example, an in-line two cylinder four stroke cycle engine may have first order and second order shaking forces but no shaking moments, an inline four cylinder engine normally exhibits a second order vertical shaking force, and a 60.degree. V-6 engine conventionally has a second order shaking couple.
First order shaking forces and moments are usually able to be balanced by the addition or removal of mass from various parts of the mechanism such as the crankshaft of an engine or the components attached thereto. This selective addition of weight may be referred to as "passive" balancing, since it requires no added mechanism. Second order shaking forces and moments are more difficult to balance because they vary at twice the speed or frequency of the engine crankshaft rotation. Therefore, an "active" balancing arrangement utilizing additional balance shafts or devices is usually required. It is well, known to obtain such balancing by providing means such as a pair of counter-rotating shafts having eccentric weights; however, because these counter-balancing shafts must run at twice the engine speed, they often cause excessive friction at the bearings and contribute to noise due to gearing or other mechanical drive arrangements between the balance shafts and the engine crankshaft.